1. Field of the Invention
This invention relates to an optical amplifier for use in an optical transmission/reception system.
2. Description of the Related Art
An optical communication system transmits an optical signal by using a light emitting source such as a semiconductor laser, and performs long-distance transmission using an optical fiber or the like as a medium. In the system, the optical signal is often amplified to increase the transmission distance or the received signal level.
FIG. 2 shows an example of the conventional optical amplifier for amplifying the optical signal. An optical fiber 21 for transmitting the optical signal is connected to a WDM (Wavelength Division Multiplexing) fiber coupler 23 via a focusing lens 22. A semiconductor laser 26 is also connected to the WDM fiber coupler 23 via a focusing lens 27. The output terminal of the WDM fiber coupler 23 is connected to an EDF (Erbium Doped Fiber) 24 made of an optical fiber doped with a rare earth element, such as Erbium, of several hundreds ppm.
An optical input signal P.sub.in is propagated through the optical fiber 21 and applied to the WDM fiber coupler 23 via the focusing lens 22. A laser beam P.sub.m for pumping the rare earth element in the EDF 24 is emitted from the semiconductor laser 26 and applied to the WDM fiber coupler 23 via the focusing lens 27. The optical input signal P.sub.in and the laser beam Pm having a different wavelength are optically coupled by the WDM fiber coupler 23 and guided in only one direction. The multiplexed beam is applied to one end of the EDF 24. The laser beam P.sub.m in the multiplexed beam pumps the rare earth element in the EDF 24. Therefore, the stimulated emission occurs by the pumped rare earth element to amplify the optical input signal P.sub.in, and the amplified optical signal is supplied from the other end of the EDF 24.
By the way, the WDM fiber coupler 23 has a complicated structure since the optical beams with the different wavelength must be multiplexed in the conventional optical amplifier so as to produce the output in only one direction. In addition, for applying the beam output from the WDM fiber coupler 23 to the EDF 24, it is necessary to connect one end of the WDM fiber coupler 23 to one end of the EDF 24 with great accuracy, i.e., the submicron order. Therefore, a highly complicated technique such as optical fiber splicing techniques must be used.
In summary, according to the conventional art, since the stimulated emission occurs by pumping the rare earth element of the optical fiber, the optical signal is amplified and transmitted. However, it is not possible to pump the rare earth element only by means of the optical signal. For this reason, a light source for pumping the rare earth element is required, and the semiconductor laser is used as the light source. Hence, for applying the optical signal and the pumping beam to the optical fiber doped with the rare earth element, the two beams must be multiplexed and guided in one direction. For this purpose, the WDM fiber coupler, which is expensive and has the complicated structure, must be used. Moreover, for coupling the WDM fiber coupler to the optical fiber, sophisticated techniques must be employed.